Method for the production of glyceric acid

ABSTRACT

Glyceric acid compounds of the formula (I)  
     R 1 R 2 C(OH)—CR 3 (OH)—COOX   (I)  
     where R 1 , R 2  and R 3  are independently hydrogen, C 1-12 -alkyl, C 6-12 -aryl, C 7-13 -alkaryl or C 7-13 -aralkyl and  
     X is hydrogen, an alkali metal, an alkaline earth metal/2 or NH 4 , are prepared by saponification of glycidic acid compounds of the formula (II)  
                 
 
     where Y is NH 2  or OR 4  in which R 4  is C 1-12 -alkyl or C 7-13 -aralkyl, with ring-opening addition of water onto the epoxide ring.  
     Preferably, Y is NH 2  and the glycidamide of the formula (II) which is used is prepared by reacting acrylonitriles of the formula (III)  
     R 1 R 2 C═CR 3 CN   (III)  
     with hydrogen peroxide.

[0001] The present invention relates to a process for preparingsubstituted or unsubstituted glyceric acid.

[0002] Glyceric acid is a chemical building block which is of interestfor chemical syntheses and an intermediate for physiologically activecompounds and amino acids.

[0003] Various methods of preparing glyceric acid are known.

[0004] DE-A 42 28 487 describes the preparation of glyceric acid byoxidation of glycerol by oxygen in the presence of catalysts. Inparticular, a catalyst comprising 1% of Ce and 5% of Pt on activatedcarbon is used, see Example 13.

[0005] JP-A 60 226 842 relates to the preparation of glyceric acid byreaction of acrylic acid with hydrogen peroxide in the presence of atungsten-containing catalyst.

[0006] U.S. Pat. No. 3,846,478 relates to the oxidation of olefiniccompounds to form glycols. Here, glyceric acid can likewise be preparedby catalytic oxidation of acrylic acid. Furthermore, the oxidation ofacrylamide to glyceramide is described in Example 14. The oxidation iscarried out using an alkali metal hypochlorite or alkaline earth metalhypochlorite in the presence of osmium tetroxide.

[0007] Disadvantages of the various process variants are anunsatisfactorily low selectivity in the oxidation of glycerol and thegreat difficulty of separating the secondary components from the desiredproduct. Initially, the separation of homogeneous catalysts from thetarget product is problematical in the oxidation of acrylic acid.

[0008] It is an object of the present invention to provide a process forpreparing substituted or unsubstituted glyceric acid which leads in highselectivity to substituted or unsubstituted glyceric acid, allowssecondary components to be separated off simply and makes it possible todispense with a catalyst.

[0009] We have found that this object is achieved by a process forpreparing glyceric acid compounds of the formula (I)

R¹R²C(OH)—CR³(OH)—COOX   (I)

[0010] where R¹, R² and R³ are independently hydrogen, C₁₋₁₂-alkyl,C₆₋₁₂-aryl, C₇₋₁₃-alkaryl or C₇₋₁₃-aralkyl and

[0011] X is hydrogen, an alkali metal, an alkaline earth metal/2 or NH₄,by saponification of glycidic acid compounds of the formula (II)

[0012] where Y is NH₂ or OR⁴ in which R⁴ is C₁₋₁₂-alkyl orC₇₋₁₃-aralkyl, with ring-opening addition of water onto the epoxidering.

[0013] “An alkaline earth metal/2” means that the alkaline earth metalions are present in accordance with their stoichiometry, so that X isone equivalent of an alkaline earth metal ion.

[0014] According to the present invention, it has been found thatglycidamides or glycidic esters can be converted in high yields andselectivities into the desired substituted or unsubstituted glycericacids by saponification with ring-opening addition of water onto theepoxide ring.

[0015] In the formulae (I) and (II), R¹, R² and R³ are preferablyindependently hydrogen, C₁₋₆-alkyl, phenyl, C₇₋₁₀-alkylphenyl orC₇₋₁₀-phenylalkyl. R¹, R² and R³ are particularly preferablyindependently hydrogen or C₁₋₆-alkyl, in particular hydrogen orC₁₋₃-alkyl. R¹, R² and R³ are particularly preferably hydrogen, so thatthe compound of the formula (I) is glyceric acid or a salt thereof andthe compound of the formula (II) is glycidamide or a glycidic ester. Inthe glycidic esters of the formula (II), R⁴ is preferably C₁₋₆-alkyl orC₇₋₁₀-phenylalkyl, particularly preferably C₁₋₃-alkyl. The glycidic acidcompound of the formula (II) which is used is particularly preferablythe amide. In particular, R¹, R² and R³ are hydrogen and Y is NH₂, sothat the glycidic acid compound of the formula (II) is glycidamide.

[0016] The preparation of glycidamide and glycidic esters is known perse. Appropriate processes are described, for example, in DE-A 19 04 077,DE-A 37 12 330 and DE-A 38 29 829.

[0017] Preferably, Y is NH₂ and the glycidamide of the formula (II)which is used is prepared by reacting acrylonitriles of the formula(III)

R¹R²C═CR³CN   (III)

[0018] with hydrogen peroxide. The preparation of glycidonitrile byreaction of acrylonitrile with hydrogen peroxide is known per se and isdescribed, for example, in DE-A 19 04 077 and DE-A 38 29 829. Forappropriate methods of preparation, reference may be made to thesedocuments. Possible uses of glycidamide mentioned in these documents arethe production of textile assistants, crop protection agents,preservatives and the production of dyes or the preparation ofcomplexing agents such as isoserine-N,N-diacetic acid, but not thepreparation of glyceric acid. DE-A 37 12 330, too, describes the use ofglycidamide for the preparation of complexing agents.

[0019] Combining the reaction of acrylonitriles with hydrogen peroxideto form glyceramides and subsequent saponification with ring-openingaddition of water onto the epoxide ring leads, in an uncomplicatedoverall process, to the desired substituted or unsubstituted glycericacid in high yields and selectivities, with starting materials whichremain in the reaction mixture or by-products being able to be separatedoff in a simple manner. In addition, it is possible to dispense with theuse of catalysts as are described in the prior art.

[0020] The saponification is preferably catalyzed by acids or bases.

[0021] After reaction of the acrylonitriles of the formula (III),unreacted acrylonitriles of the formula (III) and other by-products canbe separated from the reaction mixture by distillation.

[0022] After the reaction of the acrylonitriles of the formula (III)with hydrogen peroxide, unreacted hydrogen peroxide can also bedecomposed.

[0023] In addition, when Y=NH₂, the ammonia formed in the saponificationcan be distilled off.

[0024] Glyceric acid salts of the formula (I) formed after abase-catalyzed saponification can be converted into free glyceric acids.

[0025] Taking glyceric acid as an example, the overall process can besummarized as follows:

[0026] a) reaction of acrylonitriles with aqueous hydrogen peroxide toform glycidamide,

[0027] b) if appropriate, removal of unreacted acrylonitrile and otherby-products from step a) by distillation,

[0028] c) if necessary, decomposition of the unreacted hydrogen peroxidefrom step a),

[0029] d) saponification of the glycidamide obtained to form an alkalimetal salt or alkaline earth metal salt of glyceric acid or to formglyceric acid,

[0030] e) if appropriate, removal of the ammonia formed by distillation,

[0031] f) if desired, conversion of the alkali metal salts or alkalineearth metal salts of glyceric acid into (free) glyceric acid.

[0032] The process of the present invention can be carried outcontinuously or batchwise. The process steps (a) and (d) are preferablycarried out in a cascade of stirred vessels. However, other embodimentsare also possible. Thus, some or all of the stirred vessels can bereplaced by tube reactors.

[0033] The individual steps (a) to (f) will be described in more detailbelow for preferred embodiments using unsubstituted glyceric acid as anexample:

[0034] (a) Acrylonitrile and an aqueous hydrogen peroxide solutionhaving a strength in the range from 3 to 50% by weight, preferably from10 to 25% by weight, are reacted with one another in a molar ratio offrom 1:0.6 to 1:1.5, preferably from 1:1 to 1:1.2, at a pH of from 7 to8, preferably from 7.3 to 7.7, and a temperature of from 30° C. to 60°C., preferably from 45° C. to 55° C., in a stirred vessel. The meanresidence time in the stirred vessel is usually from 30 to 60 minutes.

[0035]  To keep the pH and the temperature constant, 5-50% strength byweight, preferably 10-20% strength by weight, aqueous sodium hydroxideis metered in. The reaction solution is mixed further in from 1 to 3,preferably 1, downstream stirred vessel(s) under identical or similar pHand temperature conditions. The mean residence time in the secondstirred vessel is usually from 15 to 90 minutes, preferably from 15 to45 minutes.

[0036] (b) The acrylonitrile not reacted in (a), which is normally from5 to 50% by weight, preferably from 10 to 30% by weight, of the amountused, and possible secondary components in step (a) which have boilingpoints lower than that of glycidamide are separated off in adistillation column which preferably operates according to the thin filmevaporation principle. The distillation column is preferably operated atthe same temperature as that in step (a) and at a pressure ofadvantageously from 50 to 160 mbar. Water is also present in thedistillate.

[0037]  The residual acrylonitrile of the reaction solution after thedistillation is from 0.1 to 4% by weight. The acrylonitrile which hasbeen separated off can, if desired, be returned to step (a) afterremoval of further substances.

[0038] (c) Step (c) is optional. Depending on how the saponification ofthe glycidamide is carried out, the decomposition of excess hydrogenperoxide before the saponification of the glycidamide may or may not benecessary. To decompose hydrogen peroxide still present, the reactionsolution is passed over a solid catalyst, preferably activated carbon.However, it is also possible to use other solid materials for destroyingthe hydrogen peroxide, for example zeolites or water-insolublemanganese, lead, vanadium or noble metal compounds. The order of steps(b) and (c) can be exchanged.

[0039] (d) The saponification of the glycidamide can be either acid- orbase-catalyzed. In the case of acid-catalyzed saponification, theglycidamide is reacted in the presence of water with an at leastequimolar amount of an acid, for example sulfuric acid or phosphoricacid. The resulting ammonium salt of the respective acid can then beseparated from the glyceric acid solution. However, the aqueousglycidamide solution is preferably passed over a heterogenous strongacid ion exchanger. An acid-catalyzed saponification forms glyceric acidas first product. For this reason, the acid-catalyzed saponification ispreferred when glyceric acid is to be prepared directly. However, shouldthe salts of glyceric acid be the target compounds, the glyceric acidformed can simply be reacted with the corresponding metal oxides orhydroxides to give the glyceric acid salts.

[0040]  In the case of base-catalyzed saponification, the glycidamidesolution is reacted with a 10-50% strength by weight, preferably 40-50%strength by weight, solution of a basic alkali metal or alkaline earthmetal compound, e.g. sodium hydroxide, potassium hydroxide or calciumhydroxide, in a molar ratio of from 1:1 to 1:1.7, preferably from 1:1.3to 1:1.4, at a temperature of from 60 to 100° C., preferably from 70 to95° C., and a pH of from 9 to 14, preferably from 11 to 12.5, in astirred vessel to give the end product. The major part of the ammoniaformed in the saponification can be removed from the product during thesaponification by application of a reduced pressure of from 200 to 700mbar or by stripping with nitrogen or another inert gas.

[0041]  A thermal treatment of the aqueous glycidamide solution obtainedfrom step (c) at temperatures of from 50 to 100° C. for from 10 minutesto 3 hours can optionally be carried out at the beginning of step (d)prior to the actual saponification reaction. The concentration of theglycidamide solution obtained from step (c) can optionally be changed byconcentrating the solution or diluting the solution with, for example,water. Furthermore, acidic or basic, homogeneous or heterogeneouscatalysts can optionally be added.

[0042]  A specific embodiment of the process provides for theglycidamide solution to be treated in the presence of a catalyst with orwithout CO₂ prior to the saponification. Such catalysts can be, forexample, metal halides, tetralkylammonium or tetralkylphosphoniumhalides and also metal oxides, metal hydrogen carbonates, metalcarbonates, metal hydroxides and metalates such as molybdate, vanadate,tungstate. The treatment of the glycidamide solution with CO₂ is carriedout at temperatures of preferably from 50° C. to 180° C. and pressuresof preferably from 1 bar to 30 bar. The glycidamide solution which hasbeen treated this way is subsequently saponified.

[0043] (e) The residual amount of the ammonia formed in the case of abasic saponification in (d) is separated off as a mixture with water ina stripping column or distillation column which preferably operatesaccording to the thin film evaporator principle. The distillation columnis preferably operated at the same temperature as that in step (d) andat a pressure of from 200 to 700 mbar, preferably from 400 to 600 mbar.This generally leaves a 2-20% strength by weight solution of theglyceric acid salt, which can be concentrated by distilling off water.

[0044] (f) If the base-catalyzed variant is chosen in step (d) and thetarget compound is glyceric acid, step (e) is followed by the conversionof the glyceric acid salt into glyceric acid.

[0045] The advantages of the process of the present invention are:

[0046] Selectivity to glycidamide in its preparation from acrylonitrileand hydrogen peroxide of >60%, based on the acrylonitrile reacted.

[0047] Simple removal of unreacted starting materials, and also, ifnecessary, removal of undesirable by-products.

[0048] The saponification of glycidamide to glyceric acid proceedsvirtually quantitatively.

[0049] The invention is illustrated by the following examples:

EXAMPLES Example 1 Preparation of Glycidamide from Acrylonitrile

[0050] (a) 76.4 g/h (1.44 mol/h) of acrylontirile (AN) and 343 g/h of a15% strength by weight aqueous hydrogen peroxide solution (correspondingto 1.51 mol of H₂O₂) together with 27 g/h of 8% strength by weightaqueous sodium hydroxide are metered at 50° C. into a stirred vessel R1(volume: 0.3 l) so that the pH was maintained at from 7.4 to 7.5. Aftera residence time of 40 minutes, the reaction mixture was transferred tothe stirred vessel R2 (volume: 0.5 l) where a further 29 g/h of 8%strength by weight aqueous sodium hydroxide were introduced to maintainthe abovementioned pH. The temperature in the stirred vessel R2 was 50°C., and the residence time was 63 minutes.

[0051] (b) In a Sambay thin film evaporator D1, which was operated at50° C. at 70 mbar, 90 g/h of distillate comprising unreactedacrylonitrile and water were distilled from the reaction mixture. 385g/h of an aqueous glycidamide solution, which according to gaschromatography had a glycidamide content of about 60% by area, wereobtained as bottoms.

Example 2 Ring-Opening Saponification of Glycidamide to Give SodiumGlycerate

[0052] 250 g of a glycidamide solution from Example 1 are saponifiedwith 613 g of 18.5% strength by weight aqueous sodium hydroxide at 100°C. and a pH of from 11.0 to 11.5 to give sodium glycerate.

We claim:
 1. A process for preparing glyceric acid compounds of theformula (I) R¹R²C(OH)—CR³(OH)—COOX   (I) where R¹, R² and R³ areindependently hydrogen, C₁₋₁₂-alkyl, C₆₋₁₂-aryl, C₇₋₁₃-alkaryl orC₇₋₁₃-aralkyl and X is hydrogen, an alkali metal, an alkaline earthmetal/2 or NH₄, by saponification of glycidic acid compounds of theformula (II)

where Y is NH₂ with ring-opening addition of water onto the epoxidering, wherein the glycidamide of the formula (II) which is used isprepared by reacting acrylonitriles of the formula (III) R¹R²C═CR³CN  (III) with hydrogen peroxide.
 2. A process as claimed in claim 1,wherein R¹, R² and R³ are independently hydrogen or C₁₋₆-alkyl.
 3. Aprocess as claimed in claim 1 or 2, wherein R¹, R² and R³ are hydrogenand Y is NH₂.
 4. A process as claimed in any of claims 1 to 3, whereinthe saponification is acid- or base-catalyzed.
 5. A process as claimedin any of claims 1 to 4, wherein unreacted acrylonitriles of the formula(III) and other by-products are separated from the reaction mixture bydistillation after the reaction of the acrylonitriles of the formula(III) with hydrogen peroxide.
 6. A process as claimed in any of claims 1to 5, wherein unreacted hydrogen peroxide is decomposed after thereaction of the acrylonitriles of the formula (III) with hydrogenperoxide.
 7. A process as claimed in any of claims 1 to 6, wherein theammonia formed in the saponification when Y═NH₂ is distilled off.
 8. Aprocess as claimed in any of claims 1 to 7, wherein glyceric acid saltsof the formula (I) formed in a base-catalyzed saponification areconverted into free glyceric acids.